Count Rumford, actually an American by the name of Benjamin Thompson, gained fame by rebuilding more than 500 fireplaces in England. It became a matter of pride to have a fireplace constructed by his masons.
In the 80’s, as a journeyman mason, I began to study and build his fireplaces which became a real problem with the Montgomery County, Maryland building department. The Rumford breaks all the rules. In the end, I convinced them to permit the shallow firebox in trade for a deeper and wider hearth.
Another challenge to constructing a Rumford was the cast iron dampers were too big. I was able to overcome this by bringing the top of the firebox back forward of the damper flange, but if I had it to do again I would use a chimney top damper.
Anyone that I ever built a Rumford for will happily tell you that the heat they produce will drive you from the room.
Before we consider the design of the Rumford it would be helpful to understand what are the most common problems with a masonry fireplace.
What causes a masonry fireplace to smoke?
- An insufficient throat area. If there is not a sufficient space just above and behind the fireplace opening and below the damper for smoke to collect and roll prior to passing through the throat it will come back into the room. The height of the opening is not critical, but this area behind the top edge of the opeining is.
A flue that is too large. That sounds crazy, but bear with me. Smoke is actually heavier than air. As smoke cools, it becomes heavy and the moisture (sap) in it begins to condensate on the sides of the flue in the form of creosote and glazing.
In the 80’s I constructed a redical masonry wood stove chimney flue under the direction of Professor Jess Brown from the pyrotechnics department of Auburn University. The wood stove exhaust was an 8" diameter. We used round terra cotta flues all the way up and surrounded them with perlite insulation (good to 2000 degrees) Three years of monitoring revealed that the flue never needed cleaning. All heat and creosote remained in gas form until released from the top of the flue.
Taller objects near the flue. The top of a tree or roofline that is near and taller that the chimney can cause wind to curl over and down into the flue.
What are the unique and successful features of the Rumford?
- The fireplace opening was huge by todays standards. Most masons would look at a Rumford and conclude that it’s going to smoke, but it doesn’t.
- All firebox surfaces were angled to radiate the heat directly into the room.
- The throat was a mere 4", just large enough to prevent smoking while minimizing heat loss.
- The smoke shelf was just large enough to permit the passage of exhaust and return air.
- A plumb bob held in the center of the flue also hits the center of the firebox floor. (Heat rose directly up without any restriction.)
Unfortunately, to meet todays safety concerns and standards the masonry fireplace has returned to the inefficient dark ages.
Next: Why brick veneer was falling out of high rise buildings onto parked cars in Texas.